In many aircraft, gas turbine engines perform a number of functions, including providing propulsion for the aircraft and driving various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical and/or pneumatic power. Such engines may include auxiliary power units (APU) that supplement main propulsion engines by providing electrical and/or pneumatic power.
A gas turbine engine typically includes a compressor section, a combustion section, and a turbine section. The compressor section compresses air provided at a forward end of the gas turbine engine and provides the compressed air to the compressor stage. Fuel is added to the compressed air, and the resulting mixture is ignited within the combustion section to produce combustion gases. The combustion gases are directed to the turbine section, which extracts energy from the combustion gases to provide the motive force necessary to power the compressor section, such that the gas turbine engine is self-sustaining.
To start the gas turbine engine, a starter motor may be used to initially provide a motive force to the shaft connecting the compressor section to the turbine section. The starter motor may be used to increase the speed of the compressor section to a point at which the compressed air provided to the combustion section results in a fuel/air mixture that is suitable for ignition (commonly referred to as “light-off”). Typically, light-off occurs at approximately 10-20% of the operating speed of the gas turbine engine. Following light-off, the starter motor is discontinued when the gas turbine engine is self-sustaining (i.e., the power generated by the turbine section is sufficient to power the compressor section) such that the gas turbine engine does not require the motive force from the starter motor.
There are a number of issues with the engine systems that control the start-up and operation of the gas turbine engines. For example, the fuel and air must be at a particular ratio to achieve ignition. Since the speeds of the engine, and thus the compressor, are accelerating during start-up, the margin of error for introducing the appropriate amount of fuel is small. Unburned fuel reduces the efficiency of the engine and increases undesirable emissions. Other aspects of engine operation may result in less than optimal fuel consumption.
Accordingly, it is desirable to provide engine systems that operate a gas turbine engine in a more efficient manner. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.